Plane is not smooth when I render at 400 sample
You are dealing with Coplanar faces, or surfaces that are parallel and/or overlap.
They should be separated; otherwise you are going to get Z-fighting errors, Where the render engine cannot discern what is in front and what is behind and randomly renders one or the other.

Like in the real world two surfaces cannot share the same space.
If the planes are coplanar and slightly separated, but not enough for the render engine to pick up the difference, then you are dealing with precision on the depth buffer, to fix that you need to adjust the clip distance of the camera (and the viewport) so that the depth of the buffer encompasses your scene and nothing else. In other words the close clipping distance should start at the distance from the camera to the first point in the geometry, and the end of the clip distance should be at the farthest point of the objects not any further.
Use the viewport display&gt;show limits option to visualize the clip range.